Single-wall carbon nanotubes (SWNT), commonly known as “buckytubes,” have unique properties, including high strength, stiffness, thermal and electrical conductivity. SWNT are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. Background information on single-wall carbon nanotubes can be found in B. I. Yakobson and R. E. Smalley, American Scientist, Vol. 85, July–August, 1997, pp. 324–337 and Dresselhaus, et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19, (“Dresselhaus”).
Several methods of synthesizing fullerenes have developed from the condensation of vaporized carbon at high temperature. Fullerenes, such as C60 and C70, may be prepared by carbon arc methods using vaporized carbon at high temperature. Carbon nanotubes have also been produced as one of the deposits on the cathode in carbon arc processes.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus by simultaneously evaporating carbon and a small percentage of Group VIIIb transition metal from the anode of the arc discharge apparatus. These techniques allow production of only a low yield of carbon nanotubes, and the population of carbon nanotubes exhibits significant variations in structure and size.
Another method of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms (such as nickel, cobalt, or a mixture thereof) to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although certain structures may predominate. Although the laser vaporization process produce can produce improved yields of single-wall carbon nanotubes, the product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials. In addition, the laser vaporization of carbon is a high energy process.
Another way to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules dissociate on the metal particle surface and the resulting carbon atoms combine to form nanotubes. The method typically produces imperfect multi-walled carbon nanotubes, but under certain reaction conditions, can produce excellent single-wall carbon nanotubes. One example of this method involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 catalyzed by transition metal catalyst particles comprising Mo, Fe, Ni, Co, or mixtures thereof residing on a support, such as alumina. Although the method can use inexpensive feedstocks and moderate temperatures, the yield of single-wall carbon nanotubes can be low, with large amounts of other forms of carbon, such as amorphous carbon and multi-wall carbon nanotubes present in the product. The method often results in tangled carbon nanotubes and also requires the removal of the support material for many applications.
All-gas phase processes can be used to form single-wall carbon nanotubes. In one example of an all gas-phase process, single-wall carbon nanotubes are synthesized using benzene as the carbon-containing feedstock and ferrocene as the transition metal catalyst precursor. By controlling the partial pressures of benzene and ferrocene and by adding thiophene as a catalyst promoter, single-wall carbon nanotubes can be produced. However, this method suffers from simultaneous production of multi-wall carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature conditions necessary to produce high quality single-wall carbon nanotubes.
Another method for producing single-wall carbon nanotubes involves an all-gas phase method using high pressure CO as the carbon feedstock and a gaseous transition metal catalyst precursor. (“Gas Phase Nucleation and Growth of Single-Wall Carbon Nanotubes from High Pressure Carbon Monoxide,” International Pat. Publ. WO 00/26138, published May 11, 2000, incorporated by reference herein in its entirety). This method permits continuous nanotube production, and it has the potential for scale-up to produce commercial quantities of single-wall carbon nanotubes. This method is also effective in making single-wall carbon nanotubes without simultaneously making multi-wall nanotubes. Furthermore, the method produces single-wall carbon nanotubes in high purity, such that less than about 10 wt % of the carbon in the solid product is attributable to other carbon-containing species, which includes both graphitic and amorphous carbon.
All known processes for formation of single-wall nanotubes involve a transition-metal catalyst, residues of which are invariably present in the as-produced material. Likewise, these processes also entail production of varying amounts of carbon material that is not in the form of single-wall nanotubes. Hereinafter, this non-nanotube carbon material is referred to as “amorphous carbon.”
All known methods of synthesizing single-carbon nanotubes also produce a distribution of reaction products, including, but not limited to, single-wall carbon nanotubes, amorphous carbon, metallic catalyst residues, and, in some cases, multi-wall carbon nanotubes. The distribution of reaction products will vary depending on the process and the operating conditions used in the process. In addition to the distribution of reaction products, the process type and operating conditions will also produce single-wall carbon nanotubes having a particular distribution of diameters and conformations.
The diameter and conformation of single-wall carbon nanotubes can be described using the system of nomenclature described by Dresselhaus. Single-wall tubular fullerenes are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When n=m, the resultant tube is said to be of the “armchair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an armchair repeated n times. When m=0, the resultant tube is said to be of the “zig-zag” or (n, 0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig-zag pattern. Where n≠m and m≠0, the resulting tube has chirality and contains a helical twist to it, the extent of which is dependent upon the chiral angle. FIG. 1 diagrams the system of nomenclature for (n, m) nanotubes.
The electronic properties of single-wall carbon nanotubes are dependent on the conformation. For example, armchair tubes are metallic and have extremely high electrical conductivity. All single-wall carbon nanotubes can be categorized as metallic, semi-metals, or semiconducting depending on their conformation. For clarity and conciseness, both metallic tubes and semi-metal tubes will be referred to collectively as metallic nanotubes. For single-wall carbon nanotubes, about one-third of the tubes are metallic and about two-thirds are semiconducting. Metallic (n, m)-type nanotubes are those that satisfy the condition: 2n+m=3q, or n−m=3q where “q” is an integer. Metallic nanotubes are conducting with a zero band gap in energy states. Nanotubes not satisfying either condition are semiconducting and have an energy band gap. Generally, semiconducting nanotubes with smaller diameters have larger energy band gaps. Regardless of tube type, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
The particular nanotube diameter and conformation affects the physical and electronic properties of the single-wall carbon nanotube. For example, the strength, stiffness, density, crystallinity, thermal conductivity, electrical conductivity, absorption, magnetic properties, response to doping, utility as semiconductors, optical properties such as absorption and luminescence, utility as emitters and detectors, energy transfer, heat conduction, reaction to changes in pH, buffering capacity, sensitivity to a range of chemicals, contraction and expansion by electrical charge or chemical interaction, nanoporous filtration membranes and many more properties are affected by the diameter and conformation of the single-wall carbon nanotube.
The properties of a collection of a particular (n, m) selected carbon nanotube will differ from those of a mixture of single-wall carbon nanotubes that are made by the different production processes. The properties of mixtures of nanotube types represent a composite value over a distribution of property values. This composite value is generally not “average” behavior. In fact, the properties of composites can not only be inferior to, but also lacking altogether in a mixture of single-wall carbon nanotubes compared to those of a particular selected (n, m) type nanotube. Additionally, in the diameter range of single-wall carbon nanotubes, generally about 0.5 nm to about 3.5 nm, the alignment of the nanotubes to each other in closely-packed arrays, such as the well-known single-wall carbon nanotube “ropes”, can be optimized when all the nanotubes are of the same diameter, minimizing misfits between tubes of different diameter.
While a method for separating and sorting single-wall carbon nanotubes of a specific type is desired in order to capture the desired properties of the selected nanotube type or types, such a method is complicated by two major factors. First is the nanotubes' extreme lack of solubility in water and most common solvents. Second is the strong propensity of single-wall carbon nanotubes to “rope” together in bundles that are strongly held together by van der Waals forces. The roping phenomenon aggregates different types of single-wall carbon nanotubes together in aligned bundles or “ropes” and holds them together with a sizable tube-to-tube binding energy of up to about 500 eV/micron. These aggregates generally contain random mixtures of metallic and semiconducting types of nanotubes with assorted diameters. When electrically contacted while in bundled aggregates, the carbon nanotubes experience sizable perturbations from their otherwise pristine electronic structure that complicates the differentiation between different types of nanotubes. Also, attempts to exploit the chemical diversity within mixtures of nanotubes, either through sidewall functionalization or end-group derivatization have not been successful in separating nanotubes of specific conformations, but have produced largely bundles of nanotubes or nanotubes with significantly altered electronic properties.
No effective process for making single-wall carbon nanotubes is known whereby significant quantities of carbon nanotubes made are of a single (n, m) type. Furthermore, to date, no methods for separating quantities of single-wall carbon nanotubes by (n, m) type are known, and no macroscopic quantity of such single (n, m) type single-wall carbon nanotube material has been produced. Macroscopic amounts of type-sorted single-wall carbon nanotubes that would provide the broadest range of possible nanotube properties and applications are heretofore unknown.